Quaternary ammonium salts (QASs) have been known and widely used for more than half a century as disinfectants. It is now accepted that they exert their biocidal activity by an electrostatic mechanism with the cell wall of bacteria. Surfaces coated with QAS-containing polymers retained their activity over a longer period of time. Polysiloxanes with QAS groups were attractive as biocidal polymers as polysiloxanes have high chain flexibility which allows easier contact between microorganisms and QAS. QASs with long alkyl chains were good candidates for this function. Structure, density, and distribution of QASs in the polymer matrix could affect their biocidal activity. A typical coating formulation containing QAS has large number of variables including the types of QASs, their levels of addition, molecular weight of polysiloxanes, levels of catalyst, and the amount of crosslinker.